The present invention relates to a transfer conveyance unit, the rotatable transfer shaft of which comes into contact with the circumferential surface of a photoreceptor drum through a conveyance belt so that a transfer paper can be conveyed while toner on the drum surface is transferred onto the paper.
Recently, many copiers can be opened like a clam-shell in such a manner that an upper portion of the copier is separated from a lower portion with regard to a recording paper conveyance passage. In the copier mentioned above, a photoreceptor drum is provided in the upper portion of the copier, and a transfer conveyance unit is provided in the lower portion. Accordingly, this type of copier is advantageous in that jam clearance can be easily carried out when the upper portion of the copier is opened.
Recently, color copiers have come into wide use. When a color image is formed on a photoreceptor drum surface, various image forming methods are used. In a color image forming system in which toner of different colors is sequentially superimposed on the surface of a photoreceptor drum each time the photoreceptor drum is rotated, the transfer conveyance unit is sufficiently separated from the drum surface during color image formation, and after color image formation has been completed, the transfer conveyance unit comes into contact with the photoreceptor drum in order to transfer the image formed on the drum surface onto a recording paper.
When a transfer shaft and transfer belt are contacted with the photoreceptor drum in the aforementioned copier, an idle shaft side of the transfer conveyance belt unit is rotated around a drive shaft or a shaft provided close to the drive shaft, and the right and left edge portions of the transfer conveyance unit are pressed against the photoreceptor drum with two compression springs.
However, when the photoreceptor drum or the upper portion of the copier in which the photoreceptor drum is installed, are inclined, or when the transfer conveyance unit is inclined, it is difficult to make the pushing force on the right side of the transfer conveyance unit equal to that on the left side only by the action of the two compression springs. Therefore, unevenness is caused in a transferred image.
The transfer conveyance unit is provided with a transfer shaft upon which a voltage is impressed so that the transfer shaft is electrically charged. Therefore, static electricity tends to accumulate in the transfer conveyance unit, so that problems such as jamming tend to occur. In order to solve this problem, a wire is used for connecting the transfer conveyance unit to ground. However, the load given by the wire greatly affects the movements of the transfer shaft and a frame on which the transfer roller is mounted. Accordingly, there is a possibility that a difference of pushing force is caused between the right and the left portion of the transfer belt and shaft when they are contacted with the photoreceptor drum.
The present invention has been achieved to solve the aforementioned conventional problems. It is a primary object of the invention to provide a transfer conveyance unit of simple structure in which the transfer belt and transfer shaft are positively contacted with the photoreceptor drum and the contact force of the right portion and that of the left portion are balanced.